1. Field of the Invention
The present invention relates to an optical LAN (local area network) and a protocol used in the optical LAN. More particularly, the invention relates to an optical LAN in which a collision on the network is detected, and the result of collision detection is used for communication control.
2. Description of the Related Art
In a general LAN, a plural number of nodes are connected to a bus. The nodes communicate with one another through the bus. One of the buses used in the LAN is a broadcasting bus. When using the broadcasting bus, a signal sent by a node can be simultaneously received by all of the nodes. ETHERNET (Trade Mark) is well known as one type of LAN using the broadcasting bus. The protocol used in ETHERNET is called a CSMA/CD (carrier sense multiple access/collision detection) system, prescribed in IEEE 803.3. In ETHERNET, coaxial cables are used for transmission medium. The nodes are connected by the coaxial cables. A node which will send a signal checks whether or not a signal from another node is present on the coaxial cable. If it is not present, the sending node starts the signal transmission. Actually, there is the possibility that two nodes simultaneously send signals. This state is called a collision. In ETHERNET, the collision is detected in the form of a voltage level in the coaxial cable.
After detecting the collision, the node sends a jamming signal for a preset time, and then is set to a random-time stand-by mode. The jamming signal must be set to be longer than the maximum Maximum round trip time of the network, in order to broadcast the collision to all of the nodes connected to the network. The random-time stand-by mode is provided in order to avoid such a situation that a plural number of nodes fail to send, and if those nodes simultaneously start to send signals immediately after a communication channel becomes idle, the collision occurs again.
The optical communication system has gradually been used also in the LAN. In the LAN using the optical fiber as the transmission media, the number of nodes cannot be increased by simply increasing the number of taps, although it can be increased so in the LAN using the coaxial cable as transmission media.
To solve the problem, there is a proposal of a new optical communication network in which each node is provided with two separate ports, one for transmission and the other for reception, and the nodes are coupled through a star coupler. For the proposal, reference is made to E. G. RAWSON ET AL., "Fibernet: Multimode Optical Fibers for Local Computer Networks", IEEE Transaction on Communications, Vol., COM-26, No. 7, p395 (1978).
The optical communication network using the star coupler is schematically illustrated in FIG. 1. In the figure, reference numerals 26a and 26b designate optical fibers; 27, nodes; 25, a star coupler of the mixing rod type; and 24, terminals. A signal output from each node 27 is converted into a light signal by a light emitting element 22 of the node. The light signal is supplied through the optical fiber 26a associated therewith to the star coupler 25. The light signals transmitted from the nodes are all mixed by the star coupler 25, and then are distributed to the light sensing elements 23 of the nodes, through the related optical fibers 26b. The light signal is converted into an electrical signal by the related light sensing element, and supplied to the node 27. In the communication network thus arranged, a signal transmitted from one node is transmitted to all of the nodes, viz., the network has a broadcasting function. Accordingly, the communication network, which is similar to ETHERNET, can be constructed.
In the proposal, as the number of nodes coupled with the star coupler is increased, the level of a receiving signal is decreased in each node. One of the possible ways to solve the level down problem is to extend the network by additionally using star couplers and relay amplifiers. This approach suffers from another problems, however. The star coupler, when receiving a signal from a node, sends it also to the receiving port of the same node. Accordingly, a feedback loop is formed between the star couplers interconnected. If a relay amplifier is located between the star couplers, an oscillation occurs. When the star coupler is used, the number of nodes that can be connected to the network is limited to the number of terminals of one star coupler.
As described above, the signal transmitted from a node is distributed to the receiving port of the same node. This makes it difficult to detect the collision. That is, since the distribution ratios of actual passive star couplers are not uniform, it is difficult to apply the level-difference detecting method for the collision detection.
To cope with the problem, a CRV (code rule violation) method was proposed for a collision detecting system, which is to be applied for the network using the passive star coupler as shown in FIG. 2. The CRV was discussed by Oguchi et al., in their paper "Study on Arranging Collision Detecting Circuits for Optical Star Networks", The Institution of Electronics and Communication Engineers, Optics/Radio Section, National Convention Record 341, 1982.
The CRV method is constructed on the basis of the fact that in the Manchester coding system used in ETHERNET, one-bit information is expressed by two bits, that is, utilizes such redundancy of the Manchester coding system.
In the Manchester coding system, the leading edge at the central part of one period of the reference clock signal shown in FIG. 2(a), is assigned to logical "1" of data, and the trailing edge, to "0" of data. FIG. 2(b) shows an example of the Manchester code, which represents data "110111". As seen from the figure, in the normal state of the Manchester code, the duration of a H (high) level state or a L (low) level state is within one period of a reference clock signal (FIG. 2(a)).
Let us consider a case where a collision signal as shown in FIG. 2(c) collides with a transmission signal as shown in FIG. 2(b). As the result of the collision, an intensity distribution of the receiving signal takes a profile as shown in FIG. 2(d). The receiving signal, when demodulated, has a bit pattern as shown in FIG. 2(e). The H level state of which the duration exceeds one period of the reference clock signal is found in the demodulated signal. Thus, when a code (code rule violation code), which should not exist, is detected, a CRV signal as shown in FIG. 2(c) is generated. The collision signal shown in FIG. 2(c) represent phase-shifted Manchester codes. Since the nodes are not synchronized, phases where the Manchester codes are added are indefinite.
The CRV method is based on the rule that when a code (CRV code), which should not exist, is detected, it is deemed that a collision occurred. As seen from the rule, the collision is detected on the probability basis, but use of hardware properly selected will suffice for practical use.
To solve such a problem that the number of nodes that can be connected to the network is limited, the applicant of the present Patent Application proposed a new technique in Published Unexamined Japanese Patent Application No. Hei. 3-296332. In the proposed technique, of a plural number of transmission coefficients, which are for describing the transfer characteristic of a star coupler, the transfer coefficient of the signal transfer between a pair of input and output terminals of a node is set to 0, so that no feedback loop is formed when plural star couplers are combined. In an optical communication network in which star couplers are interconnected as described in the specification of the above publication, a signal that is transmitted from a node will never return to the node per se. Also in the network, the collision detecting mechanism can readily be realized in a manner that a receiving port is constantly monitored in a transmission mode, and if a signal is detected at the receiving port, it is determined that a collision has occurred.
Further, in the proposed network, a node can receive a signal from another node even if it is sending a signal. Thus, the node can concurrently perform the transmitting and receiving operations. In other words, the optical communication network in the Patent Application serves as a bidirectional bus.
A technique that a single optical fiber is used for a bidirectional communication in the network including the combination of star couplers, is also disclosed in the co-pending U.S. patent application Ser. No. 07/813,443, filed by the Applicant of the present patent application. The co-pending U.S. patent application Ser. No. 07/873,448, filed by the Applicant of the present patent application, describes that a multichannel LAN can be constructed in which, by multiplexing wavelengths in the network, a plural number of broadcasting buses using a single optical fiber as a transmission media are arranged in parallel, and that a multimedia LAN which can handle both the data communication and real-time responsible signals, such as audio and video signals, can be constructed by using the plural broadcasting buses.
For the operation of the multichannel LAN, the CSMA/CD system is theoretically discussed (by Ikebata and Okada "Multi-Channel CSMA/CD with Hybrid Load Distribution/Region Distribution Scheme", Trans. of IECE (in Japanese) (B), Vol., J70-B, No. 12, pp1466-1474 (1987)).
The bidirectional communication system rejects the use of the collision detecting method in which the node constantly monitors its receiving port, and when detecting a signal at the receiving port, it decides that the collision occurred. In the bidirectional bus, to make a bidirectional communication by using the protocol of the line competition type, one will probably encounter such a situation that just before the communication between first and second nodes will start, a third node starts to send a signal. The collision of the third party is similar to the collision with another transmission node in the unidirectional communication.
In the communication network, for example, ETHERNET, which uses broadcasting buses, a signal transmitted by a node can be received by all other nodes. This broadcasting feature is disadvantageous in securing the secrecy of communication.
To solve the problem, the Applicant of the present patent application proposed a novel technique to keep away from the collision with the third node in patent application Ser. No. Hei. 3-97405. In the technique, after sending the signal, a sending node still continues to monitor the broadcasting bus for a time period .tau.1, which is longer than a go/return propagation delay .tau.0 of the broadcasting bus. A responding node starts to return a response signal after a time .tau.2, which is longer than the time .tau.1, since the responding node receives a packet destined thereto.
In a case where the line-competition type protocol is used for communication on the communication network of the broadcasting bus, a situation where plural nodes simultaneously send signals through the broadcasting bus inevitably occurs; that is, the collision inevitably occurs. If the node starts the signal transmission after monitoring a state of the broadcasting bus, there is the probability that plural nodes, not yet knowing the signal transmission of other parties, start to send signals at time intervals each shorter than the Maximum round trip time of the broadcasting bus.
In the conventional line-competition type protocol, e.g., the CSMA/CD system, the colliding nodes send jamming signals for a preset time (substantially equal to the Maximum round trip time of the broadcasting bus). The nodes are placed to the random-time stand-by mode, and then start again the signal transmission. The transmission of the jamming signal ensures a reliable collision detection. Where the collision occurs, no effective communication is performed and only the jamming signals flow through the broadcasting bus. The send requests issued from the colliding nodes are left unremoved. The latent communication demands are accumulated in the form of the random-time stand-by.
In the present specification, a state that two nodes simultaneously send signals within the Maximum round trip time of the broadcasting bus is called a 2-node collision. States that three and four nodes simultaneously send signals are called 3-node collision and 4-node collision, respectively. The collision of three or more nodes is generally called a multiple-collision. Accordingly, the 2-node collision is not the multiple-collision. Most of the collisions occurring on the broadcasting bus is the 2-node collision. The multiple-collision rarely occurs on the broadcasting bus. This will be described.
It is assumed that sending requests (referred to as calls) are randomly generated from a plural number of nodes connected to the broadcasting bus, and that an average frequency of call occurrence per unit time is .LAMBDA.. When 1000 calls are generated every second, the average frequency .LAMBDA. is 1000 calls/sec. The phenomenon is expressed by the random process, called a Poisson distribution. The Poisson distribution is the function to provide a probability that n number of calls occur when the broadcasting bus is monitored for a preset time .tau.. n generally indicates a positive integer. The Poisson distribution is given by the following equation. EQU P.tau.(n)=e.sup.-.LAMBDA..tau. /n| (n.gtoreq.1) (1)
When n=0, the Poisson distribution is given by the following equation. EQU P.tau.(0)=e.sup.-.LAMBDA..tau. (2)
When .LAMBDA.=1000 calls/sec. (=10.sup.3 calls/sec.), the probability P.tau. that one call is observed during .tau.=50 .mu.sec is EQU P.tau.(3)=1.98.times.10.sup.-5.
The probability P.tau. that no call is observed during .tau.=50 .mu.sec. is EQU P.tau.(0)=0.951.
Where .tau. is the Maximum round trip time, if one call is observed during the time period .tau.=50 .mu.sec, then no collision occurs. If two calls are observed, 2-node collision occurs. If three calls are observed, 3-node collisions occurs. P.tau.(0) indicates that when the broadcasting bus is monitored during .tau.=50 .mu.sec, no call occurs, viz., the line is left idle.
When the probability of 2-node collision occurrence is compared with that of 3-node collision occurrence, then we have EQU P.tau.(3)/P.tau.(2)=1.60.times.10.sup.-2 =1.6%,
if .LAMBDA.=1000 calls/sec and .tau.=50 .mu.sec. The probability comparison indicates that in most cases, 2-node collision occurs, and in rare case, 3-node collision occurs. It is known that as the average call-occurrence frequency .LAMBDA. per unit time, the percentage of the 3-node collisions becomes larger. In the case of large .LAMBDA., e.g., .LAMBDA.=10.sup.4 calls/sec, P.tau.(3)/P.tau.(2)=about 20%. In the graph of FIG. 3, the abscissa represents .LAMBDA. (call/sec) and the ordinate, P.tau.(3)/P.tau.(2).
The experimental results show that the average call-occurrence frequency .LAMBDA. in ETHERNET is at most 30 calls/sec. It is also known that the peak occurrence of calls is 50 to 60 times as large as the average value per day. For this, reference is made to J. P. Snoch and J. A. Hupp, "Measured performance of an Ethernet Local Computer Network", Communications of A.C., Vol. 23, No. 12, pp711 to 729 (1980). Accordingly, it is seen that .lambda.=1000 calls/sec is approximately the instantaneous maximum value of the call occurrence frequency.
As seen from the above description, if the 2-node collision can be suppressed, degradation of the channel utilization owing to the collision can be remarkably reduced. Further, it will be understood that when the 2-node collision occurs, if which of the colliding nodes has the priority to use the broadcasting bus can be decided, the result is equivalent to the case of effectively succeeding in avoidance of the 2-node collision. In other words, if one of the two colliding nodes permits the other to use the broadcasting bus, the send request of the former node is canceled. As a result, the accumulation of the potential send requests is reduced.